Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing my new design.

The broken lines showing the device and display screen illustrate environmental structure and portions of the article. All other broken lines illustrate portions of the graphical user interface. The broken lines form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 